Video encoding processes, such as those defined by the MPEG standards and by the H.264 standard, are lossy processes. That is, such processes compress an image stream by removing redundant information. Compression techniques make quality compromises, but the quality compromises are made in ways that are minimally perceptible.
It is conventional to control the amount of detail that an encoder retains by what is conventionally referred to as a Quantization Parameter (QP). When QP is small there is a relatively small amount of compression, that is, a large amount of the detail is retained. When QP is large, there is a high rate of compress, thus, more detail is lost. With respect to bit rate, a small QP results in a high bit rate and a large QP results in a lower bit rate. Controlling the value of QP in a video system is generally termed rate control.
The rate control mechanism in a video compression system is generally designed so that the system's output can meet particular bandwidth and quality requirements. In general, in order to control bit rate, a particular QP value is determined for each picture or macro block based upon the statistical properties of the video signal and the number of bits in the output buffer.
A number of different types of rate control mechanisms are described in the publicly available technical literature. For example, a publicly available document entitled: “Proposed Draft of Adaptive Rate Control” describes a rate control mechanism that is based on what is known as a Group of Pictures (GOP). The document entitled “Proposed Draft of Adaptive Rate Control” was presented at the Joint Video Team (JVT) of ISO/IEC MPEG & ITU-T VCEG (ISO/IEC JTC1/SC29/WG11 and ITU-T SG16 Q.6) 8th Meeting: Geneva, May 20-26, 2003 (document JVT-H017). The rate control mechanism described in the above referenced paper can not achieve smooth quality on GOP boundaries. Furthermore, the rate model described in the above referenced document is quadratic and thus requires a relatively complicated calculation.
The present invention provides an improved method and system for rate control.